# Why can't $ε_r<1$ (at low frequencies)?

I've heard it said that the vacuum permittivity is the lowest possible permittivity for any substance, that no substance can have $$ε_r<1$$. Is this true? If so, why is this different from permeability, where common materials like copper and graphite have $$μ_r<1$$? Is there some connection to the nonexistence of magnetic monopoles, which is the only other asymmetry I'm aware of between electrical and magnetic phenomena?

Edit: So it seems it is possible to have $$ε_r<1$$ for sufficiently high frequencies. I'm not considering high-frequency effects here; I'm asking specifically and only about electric permittivity at DC.

I would be interested to know what causes it to be frequency-dependent, but that's for another question.

• – jinawee Apr 6 '19 at 17:07
• @jinawee That actually does answer my "can it happen" question. I wasn't considering optical frequencies as the context of this was thinking about capacitor dielectrics, which certainly won't be experiencing optical frequency signals as part of normal operation. – Hearth Apr 6 '19 at 17:13
• Check out researchgate.net/post/… – jim Apr 6 '19 at 18:03
• It appears to me that all the materials with $\epsilon$r less than one are conductive. I am pretty sure there are no practical dielectric materials with $\epsilon$r less than 1. Is your interest only in dielectric materials, or does it also extend to conductive materials? Are you wanting to build a capacitor or something? Or is this just a quest for knowledge? – mkeith Apr 6 '19 at 19:14